Some focused ion beam (FIB) columns are intended for use with ion sources that emit multiple ion species. In order to select only one of these ion species for the beam to be focused on a substrate, the FIB column will typically include a mass filter. One type of mass filter, a “Wien filter,” uses crossed electric and magnetic fields (E×B) to deflect unwanted ion species off-axis, thereby causing them to strike a mass-separation aperture and is also referred to as an “E×B filter.” The relative strengths of the electric and magnetic fields are set so that the desired ion species will pass through the mass filter undeflected, then through a mass-separation aperture, and will finally be focused on the substrate surface.
Ions pass through the Wien filter within a “physical aperture,” that is, the area enclosed by the electric and magnetic pole faces. Ideally, the magnetic field and the electric field would be perpendicular to each other throughout the entire filter volume surrounded by the electric and magnetic poles. Because the electric and magnetic fields distort toward the edges of the poles, and the fields only approach the ideal perpendicular orientation and correct field strength ratio B/E toward the center of the filter region, the “optical” aperture (i.e., the aperture within which the mass separation is usable) is often much smaller than the physical aperture but larger than the beam diameter within the mass filter. It would be desirable for the magnetic and electric poles to both extend outwards away from the beam axis well past the physical aperture so that the pole ends are away from the filter region, thereby making the fields more uniform within the physical aperture and thus enlarging the actual acceptance aperture. This is impossible, however, since the electric and magnetic poles would physically interfere with each other.
In a focused ion beam column having an intermediate crossover between the source and substrate, the small optical aperture may be acceptable because the beam is relatively small in diameter. Many focused ion beam (FIB) columns include one or more crossovers, typically between multiple lenses in the FIB column. A crossover may be generated in the column to allow a wider range of magnifications between the source and target than might be achievable in a column without a crossover. If a beam is focused to a crossover after passing through a mass filter, there may be a multiplicity of crossovers, all in roughly the same plane, where each crossover corresponds to a different charge-to-mass ratio in the beam. For example, in a silicon-gold alloy liquid metal ion source (LMIS), there would typically be crossovers for singly- and doubly charged monatomic ions of silicon and gold, as well as singly- or multiply-charged silicon and/or gold multi-atomic ions.
If a mass filter aperture having a sufficiently small aperture opening (generally larger than the diameters of the crossovers) is placed in the plane of these crossovers, then only one of these ion species will pass through the aperture to subsequently be focused on the target, while all other ion species will strike the aperture plate and thus be blocked from passing into the lower portion of the FIB column. An advantage of having a crossover is that less dispersion is required in the mass filter to fully separate the various ion species than would be the case without a crossover. The crossover at the mass filter aperture serves as the virtual source for the probe-forming optics below the mass filter. Unavoidable energy spreads in the ion beams will, however, cause a blurring of the crossover along the mass dispersion axis, potentially resulting in blurring of the focused beam at the target.
Crossovers also have disadvantages—1) electrostatic repulsions are increased as the particles are brought closer together at the crossover itself, 2) the beam is generally smaller throughout the entire column due to the crossover, increasing the space-charge repulsions, and 3) sputter damage to the mass separation aperture is increased due to the higher beam current density at the mass separation aperture plate. Electrostatic repulsion due to space-charge effects spread the beam radially (Loeffler effect) and increases the energy spread (Boersch effect), both effects tending to reduce the beam current density at the work piece surface.
In a focused ion beam column without an intermediate crossover, the beam diameters are larger and the smaller optical aperture of a typical prior art mass filter may be more of a problem. There are examples of E×B mass filters in the prior art having a wide optical aperture, but such prior art mass filters have other drawbacks. One example of such a prior art mass filter with a wider optical aperture is illustrated in “Achromatic two-stage E×B mass filter for a focused ion beam column with collimated beam”, Teichert, J., and Tiunov, M. A., Meas. Sci. Technol. 4 (1993) pp. 754-763 (see FIGS. 5-8). The electrostatic electrodes in this prior art mass filter (see FIG. 5) are much wider (the vertical dimension of the physical aperture in FIG. 5) than the electrode spacing (the horizontal dimension of the physical aperture in FIG. 5) in order to make the E-field relatively uniform over an optical aperture having roughly the horizontal and vertical dimensions of the electrode gap (i.e., the optical aperture is roughly square while the physical aperture is rectangular with a substantially larger dimension vertically). A more square physical aperture is preferred over a rectangular physical aperture since a larger portion of the area of the physical aperture is then usable for a round beam. The magnetic poles, (as a result of the wide electrodes) are necessarily relatively far apart (vertically in FIG. 5). In order to achieve a high ratio between the width (measured horizontally) of the magnetic pole pieces to the pole piece gap (vertical spacing), this prior art E×B design requires magnetic pole piece widths that are several times larger than the width of the physical aperture (see FIG. 5)—this makes the magnetic circuit relatively inefficient, requiring substantially increased permanent magnet strengths or magnetic coil excitations to achieve adequate B-field strengths in the physical aperture. Section 3 of this reference teaches the prior art design approach of separated electric and magnetic poles, wherein both the electrodes and magnetic poles are electrical conductors and none are electrically resistive. Teichert and Tiunov discuss the optimization of mass separation characteristics of their E×B filter by adjustment of the widths and gaps within the constraints of this prior art design approach.
U.S. Pat. No. 4,929,839 to Parker and Robinson for a “Focused Ion Beam Column” describes an all-electrostatic FIB column with two three-element electrostatic lenses, capable of focusing ion beams on a substrate over an energy range from 4 keV to 150 keV. The column contains an E×B Wien filter and an electrostatic blanker. A single electrostatic octupole below the final lens scans the beam on the substrate. A large working distance below the final lens enables the potential insertion of additional optical elements for charge neutralization and/or collection of secondary ions for imaging or SIMS. The beam energy at the Wien filter is relatively high, 30 keV, thereby reducing the chromatic aberrations induced by mass separation. An intermediate crossover is formed in the column by the first (upper) lens—this crossover is usually at the plane of the aperture below the Wien filter.
U.S. Pat. No. 4,789,787 to Parker for a “Wien Filter Design” describes a Wien E×B mass or velocity filter in which the optical aperture is a substantial fraction of the size of the physical aperture. The patent describes the use of ferrite magnetic pole pieces for generating the magnetic field generally perpendicular to the direction of the ion beam passing through the Wien filter. An electric current flowing through the ferrite enables the generation of a uniform electric field between the electric pole pieces, with no negative effects due to the magnetic pole pieces. Each of the two ferrite pole pieces is clamped in physical and electrical contact with both of two electric pole pieces, which may be comprised of stainless steel, or any other non-magnetic electrically-conductive material. The voltage applied across the electrodes to generate the electric field causes an electric current to flow through the ferrite which is in contact with the electric pole pieces. There are no separate electrical connections to provide a voltage to the ferrite independent of the voltages applied to the two electrodes. The current straightens the electric field lines in the Wien filter. The described Wien filter is thus characterized as having contact between the electric and magnetic pole pieces in order to accomplish the electric field enclosure process arising from the currents flowing in the two ferrite pole pieces parallel to the direction of the electric field between the two electric pole pieces in the Wien filter.
By small variations in the voltages applied to the electric pole pieces, it is possible to steer the beam side-to-side along the electric field axis. The Wien filter as described in this patent has no capability for applying deflecting fields along the magnetic field axis. In addition there is no capability for applying a quadrupole electric field to stigmate the beam.